In the past, there have been many different types of non-return valves for flue gas venting. During the normal operation, water often condenses in the chimney. To accommodate for this condensate, prior art devices provide a condensate trap. However, many prior art designs require pre-filling of the condensate trap to prevent hazardous flue gas from entering the appliance or property. Furthermore, such condensate traps cannot be permitted to become dry, otherwise they would no longer be operational. Moreover, many prior art devices do not facilitate servicing, such as cleaning and replacing, of the dampers or floats without having to dismount the non-return valve from the piping system of the chimney.